The complement system is a complex group of proteins in blood that, in concert with antibodies and other factors, plays an important role as a mediator of immune, allergic, immunochemical and immunopathological reactions. Activation of the complement system can result in a wide range of reactions such as opsonization and lysis of various kinds of cells, bacteria and protozoa, inactivation of viruses, and the direct mediation of inflammatory processes. Through the hormone-like activity of several of its components, the complement system can recruit and enlist the participation of other humoral and cellular effector systems. These in turn can induce directed migration of leukocytes, trigger histamine release from mast cells, and stimulate the release of lysosomal constituents from phagocytes.
The complement system consists of at least twenty distinct plasma proteins capable of interacting with each other, with antibodies, and with cell membranes. Many of these proteins, when activated, combine with other proteins to form enzymes to cleave and activate still other proteins in the system. The sequential activation of these proteins, known as the complement cascade, follows two main pathways; the classical pathway and the alternative pathway. Both pathways converge at C3 and use a common terminal trunk which leads to cell lysis, bacterial opsonization and lysis, or viral inactivation.
The classical pathway can be activated by antigen-antibody complexes, aggregated immunoglobulins and non-immunological substances such as DNA and trypsin-like enzymes. The classical pathway of activation involves, successively, four components denominated C1, C4, C2 and C3. These components can be grouped into two functional units: C1 or recognition unit; and C4, C2, and C3 or activation unit. Five additional components denominated C5, C6, C7, C8, and C9 define the membrane attack complex (MAC) forming the terminal trunk common to both pathways that leads to cell lysis. The alternate pathway utilizes Factor B and bypasses the C1-C4-C2 steps, activating at C3.
The classical pathway begins with the C1-complex, which consists of one molecule of C1q and two molecules of both C1r and C1s. Activation of the C1-complex is triggered either by C1q's binding to antibodies from classes M and G, complexed with antigens, or by binding of C1q to the surface of a pathogen. Both C1r and C1s are serine proteases. Binding of C1q leads to conformational changes in the C1q molecule, which in turn leads to the activation of the two C1r molecules, followed by activation of the C1s molecules. In order to prevent spontaneous activation of this cascade, C1r and C1s are inhibited by C1-inhibitor, a serine protease inhibitor. Once activated, the C1-complex binds to and cleaves C2 and C4, producing C2a and C4b. C2a and C4b then bind to form a C4b2a complex, known as C3-convertase. Production of C3-convertase leads to cleavage of C3 into C3a and C3b; the latter joins with C2a and C4b (the C3 convertase) to make C5 convertase, which is the initial component of MAC.
Study and measurement of the activation of a complement pathway can provide an indication of many possible biological disorders. The complement pathway has been implicated in the pathogenesis or symptomatology of a broad spectrum of human diseases and pathologic conditions. Such diseases include immune complex diseases of several types, autoimmune diseases, in particular systemic lupus erythematosus, and infectious diseases, such as those found to be involved in infections with gram negative bacteria, viruses, parasites, fungi, and various dermatologic, renal, and hematologic diseases. Some disorders may be due to insufficient complement in the patient.
There remains a need in the field for methods for detecting complement fixing antibodies in a patient sample in a manner that is rapid, sensitive, and specific, particularly with respect to the ability to differentiate accurately and definitively among specific antigens.
The present invention addresses these needs.